1. Field of the Invention
The present invention relates generally to lead frames used for electrical connection to a semiconductor device. More specifically, the present invention relates to an enhanced lead frame having one or more power source or ground leads of a conventional lead frame extending along a portion of the periphery of the semiconductor device.
2. State of the Art
Well known types of semiconductor devices are connected to a component known as lead frames and subsequently encapsulated in plastic for use in a wide variety of applications. The lead frame is typically formed from a single, continuous sheet of metal, typically by metal stamping or chemical etching operations. A "conventional" lead frame usually includes an outer supporting frame, a central semiconductor device support pad (paddle), and a plurality of lead fingers, each lead finger having, in turn, a terminal bonding portion near the central semiconductor device supporting pad. In the assembly of semiconductor devices utilizing such lead frames, a semiconductor device is secured to the central supporting pad, a paddle, (such as by a solder or epoxy die-attach, although a double-sided adhesive tape-type attach has also been suggested in the art). The lead fingers are electrically connected to bond pads on the semiconductor device using fine wires. In a standard wire bonding process, the bond wires are attached, one at a time, from each bond pad on the semiconductor device and to a corresponding lead finger of the lead frame. The bond wires are generally attached through one of three industry-standard wire bonding techniques: ultrasonic bonding--using a combination of pressure and ultrasonic vibration bursts to form a metallurgical cold weld; thermocompression bonding--using a combination of pressure and elevated temperature to form a weld; and thermosonic bonding--using a combination of pressure, elevated temperature, and ultrasonic vibration bursts. After the wire bonds between the contact pads of the semiconductor device and the lead fingers are made, the semiconductor device and wire bonds are typically encapsulated in plastic using a transfer or injection molding process. Finally, the rails of the outer supporting frame of the lead frame are removed leaving portions of the lead fingers extending beyond the encapsulated semiconductor device.
One common variation on this arrangement is to eliminate the die support pad or paddle and attach the semiconductor device to the lead fingers of the lead frame using an alpha barrier such as a polyamide tape, for example Kapton.TM. tape. In such an arrangement, a so-called "leads over chip" arrangement ("LOC"), a plurality of lead fingers extend over the active surface of a semiconductor device toward one or more lines of bond pads wherein bond wires make the electrical connection between the lead fingers and the bond pads. Examples of such LOC configurations are shown in U.S. Pat. No. 4,862,245 to Pashby and U.S. Pat. No. 5,286,679 to Farnsworth et al. assigned to the assignee of the present invention.
In a conventional lead frame configuration, some of the lead fingers carry a signal to the semiconductor device while others provide a power source or a ground. In an LOC frame configuration, the lead fingers likewise provide a signal to the semiconductor device but the power source and ground are typically provided by bus bars. The bus bars typically form elongated contact portions in close proximity to the one or more lines of bond pads on the active surface of the semiconductor device, each bus bar having the contact portion thereof extending perpendicular to the other lead fingers and over the active surface of the semiconductor device.
It is often necessary to change the design and internal configuration of a semiconductor device as specification requirements change and as advancements and improvements are made in technology. As these changes are made, it may become necessary to relocate the position of the bond pads that will receive power or provide a ground and also to add additional power source and ground bond pads. This situation causes difficulties because there is often a limited number of lead fingers of a lead frame available to provide for signals, a power source, and a ground. That is, adding another power source or ground bond site at a different location on the semiconductor device may not be possible if there is not an available lead finger of the lead frame. Alternatively, it may be necessary to maintain the position of the bond pad and route the power source and ground internally in the semiconductor device. However, internal power and ground buses add to the size of the semiconductor device and decrease its speed and performance, making this alternative device design often unacceptable. In addition, changes in the semiconductor device design can require changes in production equipment and tooling, such as wire bonding and molding equipment, which are very costly.
Therefore, it would be advantageous to develop a lead frame configuration that would conserve the limited number of lead fingers, that would help improve the speed of the semiconductor device, that would help accommodate varying sizes of semiconductor devices, and that would accommodate varying bond pad locations on semiconductor devices. In addition, it would be advantageous to develop a lead frame that would accommodate changes in semiconductor device design while taking advantage of current tooling such as molding equipment.
The use of bus bars has been directed at LOC lead frame configurations and is illustrated in U.S. Pat. Nos. 4,862,245 and 5,286,679. However, such methods do not address the problem of limited leads on conventionally configured lead frames having lead fingers located about the periphery of the semiconductor device which many manufacturers of semiconductor devices are equipped to assemble, wire bond, and encapsulate such semiconductor devices thereto. The cost of converting or replacing equipment, especially wire bonding and molding equipment, to produce LOC lead frame configurations, rather than conventional lead frame configurations, can be very costly.
The use of a metallic film with the semiconductor device to provide contact with the power supply is disclosed in U.S. Pat. No. 5,497,032 to Tsuji et al. The metallic film may be divided into several separate zones in order to provide contact with different power supply systems and grounds. However, such a process requires the additional parts of the film and an insulator to separate the lead frame from the film. Also, an additional step of mounting the semiconductor device to the film is required.
The present invention is directed to an enhanced lead frame having one or more power source or ground leads of a conventional lead frame extending along a portion of the periphery of the semiconductor device.